Energy systems — AQA A-Level Physical Education

The energy systems topic covers how the body resynthesises ATP (adenosine triphosphate) to provide energy for muscle contraction. It examines three systems: the ATP-PC system (immediate), the anaerobic glycolytic system (short-term), and the aerobic system (long-term). Each system differs in fuel source, duration, intensity, ATP yield, and by-products, and they interact during physical activity.

This topic is fundamental to understanding fatigue, recovery, and training adaptations in A-Level PE. It links to other areas such as muscle contraction, sports performance, and training programmes. Exam questions often require application to sporting examples, analysis of data, and evaluation of energy system contribution.

• →ATP is the only usable form of energy for muscle contraction; it is broken down into ADP + Pi, releasing energy, and must be resynthesised continuously.
• →The ATP-PC system uses phosphocreatine to rapidly resynthesise ATP anaerobically for up to 10 seconds, producing creatine and phosphate as by-products.
• →The anaerobic glycolytic system breaks down glucose to pyruvate, producing 2 ATP per glucose and lactic acid as a by-product, lasting up to 2 minutes.
• →The aerobic system uses oxygen to break down glucose, fats, or proteins, producing large amounts of ATP (up to 38 per glucose) with carbon dioxide and water as by-products.
• →The interplay of energy systems means all three contribute to ATP resynthesis at any time, but the dominant system depends on exercise intensity and duration.

• Identifies that ATP is the immediate energy currency and that stored ATP is limited, so it must be resynthesised continuously.
• States that the ATP-PC (phosphocreatine) system is anaerobic, uses creatine phosphate to rebuild ATP and dominates the first few seconds of maximal exercise.
• States that anaerobic glycolysis breaks down glucose or glycogen to pyruvate, producing ATP quickly and forming lactate when oxygen supply is insufficient.
• States that the aerobic system uses oxygen to break down glucose, glycogen or fat in the mitochondria, yielding large amounts of ATP for prolonged submaximal work.
• Explains that during recovery, oxygen consumption stays elevated above resting level (EPOC) to restore ATP and PC stores, clear lactate and replace oxygen.
• Applies the correct dominant system to a named intensity, for example ATP-PC for a 100 m sprint and aerobic for a marathon.
• Recognises that the systems overlap and that the dominant contributor changes as intensity and duration change.
• States that ATP is the universal energy currency and that its breakdown to ADP and inorganic phosphate releases energy for muscle contraction.
• States that ATP must be resynthesised from ADP and inorganic phosphate, and that this requires energy from food fuels.
• Describes the ATP-PC system as a fast, anaerobic transfer using phosphocreatine to rebuild ATP for short maximal bursts.
• Describes anaerobic glycolysis as the breakdown of glucose or glycogen to pyruvate, producing ATP without oxygen and forming lactate.
• Describes the aerobic system as oxygen-dependent oxidation of glucose, glycogen or fat in the mitochondria, producing carbon dioxide, water and large amounts of ATP.
• Identifies the by-products and limitations of each system, such as fatigue from lactate accumulation and the slow start of aerobic transfer.
• Applies the correct transfer route to a named activity or intensity.
• Glycolysis occurs in the sarcoplasm and converts glucose or glycogen to pyruvate, giving a small net ATP yield and reduced NAD.
• Pyruvate is converted to acetyl coenzyme A before entering the Krebs (citric acid) cycle in the mitochondrial matrix.
• The Krebs cycle releases carbon dioxide, reduces NAD and FAD, and produces a small amount of ATP.
• Beta oxidation breaks fatty acids into acetyl coenzyme A units that enter the Krebs cycle.
• The electron transport chain uses reduced NAD and FAD to generate most ATP, with oxygen as the final electron acceptor forming water.
• The aerobic system has a high ATP yield but a slower rate than anaerobic systems, suiting prolonged submaximal exercise.
• The ATP-PC system uses phosphocreatine breakdown, catalysed by creatine kinase, to resynthesise ATP from ADP and inorganic phosphate.
• The ATP-PC system is very fast but limited by small phosphocreatine stores, lasting only a few seconds.
• The anaerobic glycolytic system breaks down glucose or glycogen to pyruvate in the sarcoplasm without oxygen.
• Pyruvate conversion to lactate is associated with hydrogen ion accumulation, which contributes to fatigue and limits duration.
• Anaerobic systems have a low ATP yield but a fast rate, suiting high-intensity, short-duration exercise.
• Anaerobic glycolysis supplies ATP for roughly ten seconds to about two minutes of maximal work.
• The three energy systems are ATP-PC, anaerobic glycolysis and the aerobic system.
• All three systems contribute at the same time; the continuum shows their relative contribution.
• ATP-PC dominates very short, very high-intensity efforts such as a 100 m sprint.
• Anaerobic glycolysis contributes increasingly as high-intensity effort continues beyond the initial seconds.
• The aerobic system dominates prolonged submaximal activity such as a marathon.
• The dominant system shifts with both intensity and duration, not duration alone.
• Intensity and duration must be considered together when judging the dominant energy system.
• Very short maximal efforts such as a 100 m sprint are dominated by ATP-PC.
• High-intensity efforts lasting longer, such as a 400 m race, involve strong anaerobic glycolysis contribution.
• Continuous high-intensity events like a 1500 m race rely on the aerobic system for sustained work alongside anaerobic systems.
• Prolonged submaximal activity such as a marathon is dominated by the aerobic system.
• Intermittent activities such as team games require interaction of all systems, with the aerobic system aiding recovery between efforts.
• Recovery periods allow partial resynthesis of ATP-PC and clearance of metabolic by-products.
• Slow twitch (type I) fibres generate ATP mainly by aerobic respiration, using oxygen, with high mitochondrial density and capillary supply.
• Fast twitch (type II) fibres generate ATP mainly by anaerobic glycolysis and the phosphocreatine system, with fewer mitochondria.
• Aerobic ATP generation has a higher yield per glucose molecule but a slower rate than anaerobic generation.
• Type IIa fibres are more oxidative than type IIb/IIx fibres, which are the most glycolytic.
• Slow twitch fibres are more resistant to fatigue; fast twitch fibres fatigue quickly.
• Myoglobin content is higher in slow twitch fibres, supporting oxygen storage and aerobic ATP production.
• Short duration, high intensity exercise relies on anaerobic energy transfer because oxygen supply cannot meet demand quickly enough.
• The ATP-PC (phosphocreatine) system regenerates ATP rapidly from creatine phosphate and ADP for approximately the first 10 seconds.
• Anaerobic glycolysis breaks down glucose to pyruvate, forming lactate, and regenerates ATP for up to about two minutes.
• Stored ATP is used first but is exhausted within a few seconds.
• These anaerobic systems produce ATP quickly but lead to fatigue, limiting performance duration.
• Aerobic respiration contributes little during maximal short duration exercise because it is slower to meet ATP demand.
• Anaerobic means ATP is resynthesised without oxygen.
• The anaerobic system has two components: the ATP-PC (phosphocreatine) system and the lactic acid (anaerobic glycolysis) system.
• ATP is the usable energy currency; it splits into ADP and inorganic phosphate, releasing energy for muscle contraction.
• The ATP-PC system uses stored phosphocreatine to rebuild ATP rapidly for short, maximal bursts.
• The lactic acid system breaks down glucose to pyruvate and then lactate, supplying ATP for high-intensity work lasting roughly ten seconds to about two minutes.
• Anaerobic contributions dominate at the start of exercise and during high-intensity efforts; oxygen is used afterwards to repay the oxygen deficit.
• The ATP-PC system resynthesises ATP using phosphocreatine stored in muscle.
• ATP splits to ADP and inorganic phosphate, releasing energy for muscle contraction.
• Creatine kinase breaks down phosphocreatine into creatine and phosphate, and the energy released rebuilds ADP to ATP.
• It is the fastest ATP-producing system, is anaerobic and produces no lactate.
• Phosphocreatine stores are limited, so the system supports only a few seconds of maximal work.
• Recovery rebuilds phosphocreatine using oxygen, so repeated maximal efforts decline until stores are restored.
• Glycolysis breaks down glucose or glycogen anaerobically to pyruvate, producing a small net ATP yield before lactate formation.
• Without sufficient oxygen, pyruvate is converted to lactate, and lactate accumulation lowers muscle pH and impairs enzyme activity and contraction.
• Lactate threshold is the intensity at which blood lactate rises faster than clearance; OBLA is the point where lactate reaches a defined concentration such as 4 mmol·dm⁻³.
• Lactate producing capacity supports high-intensity sprint and power performance, for example 400 m or repeated sprints lasting roughly ten seconds to two minutes.
• The aerobic system dominates during long duration, lower intensity exercise because oxygen supply meets demand.
• Pyruvate enters the Krebs cycle and electron transport chain, producing large ATP yields from glucose, glycogen and fats.
• Fat oxidation increases as duration extends, sparing glycogen stores and supporting sustained work.
• Intensity stays below the lactate threshold, so lactate is cleared as quickly as it is produced and does not accumulate.
• Aerobic resynthesis of ATP requires oxygen and occurs mainly in the mitochondria.
• Glycolysis breaks glucose into pyruvate in the sarcoplasm and yields a small amount of ATP.
• The link reaction converts pyruvate into acetylcoenzyme A, which enters the Krebs cycle.
• Hydrogen removed in the Krebs cycle provides electrons that pass along the electron transport chain.
• Hydrogen ions are pumped across the membrane and flow through ATP synthase to produce ATP.
• Oxygen is the final electron acceptor, combining with hydrogen to form water.
• Oxidative phosphorylation produces the largest ATP yield of the three energy systems.
• Fat oxidation yields more ATP per gram than carbohydrate but at a slower rate.
• The aerobic system predominates during prolonged submaximal exercise.
• Oxygen deficit is the shortfall between actual oxygen consumption and the oxygen that aerobic metabolism would have required from the start of exercise.
• At the onset of exercise, anaerobic systems such as ATP-PC and anaerobic glycolysis supply ATP before aerobic respiration accelerates.
• Submaximal exercise allows oxygen consumption to plateau at a steady state, so the oxygen deficit is relatively small.
• Maximal exercise drives oxygen consumption to VO2 max, and the oxygen deficit is larger because anaerobic contribution is greater.
• EPOC is the elevated oxygen consumption after exercise that helps restore the body to its pre-exercise state.
• EPOC supports replenishment of phosphagen stores, lactate clearance and restoration of myoglobin and blood oxygen stores.
• Trained performers reach steady state faster and have a higher VO2 max, which reduces the relative oxygen deficit at a given intensity.
• EPOC is the volume of oxygen consumed above resting baseline after exercise ends.
• The fast (alactacid) component lasts about 2–3 minutes and replenishes PC stores and myoglobin oxygen.
• The slow (lactacid) component lasts longer and is associated with lactate removal, elevated heart rate, ventilation and body temperature.
• EPOC can be expressed as a volume in litres (L) or millilitres per kilogram (ml·kg⁻¹).
• EPOC is not the same as the oxygen deficit at the start of exercise.
• VO₂ max is the maximum rate of oxygen uptake, transport and use during exercise, expressed in L/min (absolute) or ml·kg⁻¹·min⁻¹ (relative).
• Genetic makeup sets an individual's upper limit for VO₂ max.
• Training, especially aerobic training, can increase VO₂ max by improving cardiac output, stroke volume and capillary density.
• Age and sex affect typical VO₂ max values: it tends to be higher in young adults and often higher in males than females.
• Body composition, particularly a high proportion of lean muscle mass, is associated with a higher VO₂ max.
• Cardiovascular and respiratory efficiency, including haemoglobin concentration and oxygen delivery, influences VO₂ max.
• Energy expenditure is the energy used by the body for ATP resynthesis, measured in kJ or kcal (1 kcal ≈ 4.2 kJ).
• Indirect calorimetry estimates energy expenditure by measuring oxygen uptake and carbon dioxide production to calculate the respiratory exchange ratio (RER).
• Lactate sampling measures blood lactate concentration to assess anaerobic energy expenditure and identify the lactate threshold or OBLA.
• VO₂ max tests evaluate maximum aerobic energy expenditure and can be conducted via laboratory gas analysis or field tests.
• These measurements are used to monitor training intensity, evaluate fitness levels and estimate the energy cost of exercise.
• Indirect calorimetry estimates energy expenditure from oxygen uptake (VO₂) and carbon dioxide production (VCO₂) rather than from heat loss.
• The respiratory exchange ratio (RER) is calculated as VCO₂ ÷ VO₂ and indicates the substrate being oxidised.
• An RER near 0.7 indicates mainly fat oxidation; near 1.0 indicates mainly carbohydrate; above 1.0 indicates a large anaerobic contribution.
• Energy expenditure is estimated by multiplying oxygen uptake by the energy equivalent of oxygen (about 20 kJ per litre for fat and about 21 kJ per litre for carbohydrate).
• The method is accurate in laboratory conditions but needs specialist equipment and a mouthpiece or mask that can alter breathing.
• Lactate sampling measures blood lactate concentration, usually from a fingertip or earlobe blood sample.
• A portable lactate analyser is used to read the concentration, often in mmol·dm⁻³.
• Lactate rises when the anaerobic glycolytic (lactate) system contributes more to ATP resynthesis than lactate can be cleared.
• The lactate threshold is the exercise intensity at which blood lactate begins to accumulate above resting levels.
• Sampling timing (during exercise versus recovery) affects the reading, so results are interpreted against the protocol used.
• Lactate sampling can be used to monitor training intensity and to estimate the lactate threshold.
• A VO₂ max test measures the maximum rate of oxygen uptake, transport and use during maximal exercise.
• It is typically performed on a treadmill or cycle ergometer with progressive intensity increases until exhaustion.
• Expired air is analysed with a gas analyser while the performer breathes through a mouthpiece or mask.
• VO₂ max is identified when oxygen uptake plateaus despite further increases in work rate, or when secondary criteria are met.
• Results are expressed relative to body mass, for example in cm³·kg⁻¹·min⁻¹, or as an absolute value in dm³·min⁻¹.
• VO₂ max is an indicator of aerobic power and can be used to assess and monitor aerobic fitness.
• RER = VCO₂ ÷ VO₂, comparing carbon dioxide produced with oxygen consumed.
• A value near 0.7 indicates predominantly fat oxidation at rest or low intensity.
• A value near 1.0 indicates predominantly carbohydrate oxidation at higher intensity.
• Values slightly above 1.0 can reflect hyperventilation and buffering rather than pure fuel use.
• RER shifts with exercise intensity because the fuel mixture changes.
• RER is a whole-body estimate of substrate use, not a direct measure of one muscle's fuel.
• Continuous training at low to moderate intensity primarily develops the aerobic system.
• Interval and high-intensity interval training stress the ATP-PC and glycolytic systems.
• Adaptations include increased phosphocreatine stores, improved buffering and greater lactate tolerance.
• Aerobic adaptations include improved oxygen delivery and increased mitochondrial density.
• Training method should be matched to the dominant energy system of the performer's event.
• Recovery duration and work-to-rest ratio determine which system is predominantly stressed.
• Altitude training involves exposure to high altitude, where the partial pressure of oxygen is lower than at sea level.
• Reduced oxygen availability creates hypoxic stress that the body detects.
• The kidneys release more erythropoietin (EPO) in response to hypoxia.
• EPO stimulates the bone marrow to increase red blood cell production.
• A greater red blood cell count raises the oxygen-carrying capacity of the blood.
• Improved oxygen transport benefits the aerobic energy system and endurance performance.
• Live high, train low allows blood adaptations while maintaining high-intensity training near sea level.
• HIIT alternates short periods of high-intensity work with brief recovery periods.
• Work intervals are performed at maximal or near-maximal intensity.
• The ATP-PC system and anaerobic glycolysis supply much of the energy during work intervals.
• Recovery intervals allow partial resynthesis of phosphocreatine and removal of by-products.
• HIIT can improve both anaerobic power and aerobic capacity.
• HIIT is time-efficient compared with longer continuous steady-state training.
• Plyometrics uses the stretch-shortening cycle, involving an eccentric phase, an amortisation phase and a concentric phase.
• Elastic energy is stored in the muscle and tendon during the eccentric phase and released during the concentric phase.
• The stretch reflex increases muscle activation, adding to the force produced.
• A short amortisation phase allows more stored elastic energy to be reused, increasing power output.
• Plyometric work is short and intense, so it is fuelled mainly by the ATP-PC (phosphocreatine) energy system.
• Examples include depth jumps, bounding and hurdle hops, which develop explosive power for jumping and sprinting.
• Speed is the ability to cover a given distance in the shortest possible time.
• Agility is the ability to change direction quickly and under control while maintaining balance.
• Quickness is the ability to react and move explosively over very short distances.
• SAQ drills include ladder runs, cone shuttles and reaction sprints.
• Short, intense SAQ efforts are fuelled mainly by the ATP-PC energy system, with anaerobic glycolysis contributing during repeated efforts.
• SAQ training develops neuromuscular coordination, reaction time and rapid force production, which transfer to game situations.

• 💡Link each energy system to a specific intensity and duration so your answer is applied, not just descriptive.
• 💡Use correct terminology such as ATP resynthesis, phosphocreatine, anaerobic glycolysis, mitochondria and EPOC.
• 💡When a question asks about recovery, mention oxygen debt or EPOC and what is being restored, not just that the performer rests.
• 💡Use the correct equation direction: ATP breaks down to ADP and inorganic phosphate, releasing energy, and is rebuilt from ADP and inorganic phosphate.
• 💡Name the site of each process, such as the sarcoplasm for glycolysis and the mitochondria for aerobic oxidation.
• 💡Link each energy transfer route to a sporting example so the answer shows application as well as knowledge.
• 💡Link each stage to its location (sarcoplasm, mitochondrial matrix, inner mitochondrial membrane) to secure structure marks.
• 💡State the role of oxygen precisely as the final electron acceptor forming water, not as a fuel.
• 💡Compare aerobic yield and rate with anaerobic systems when a question asks why aerobic dominates in endurance events.
• 💡Use correct terminology such as acetyl coenzyme A, reduced NAD and oxidative phosphorylation rather than vague wording.
• 💡Name the enzyme creatine kinase when describing ATP-PC breakdown to show precise knowledge.
• 💡Distinguish clearly between lactate as a by-product and hydrogen ions as the cause of fatigue.
• 💡Link each system to a sporting example, such as a 100 m sprint for ATP-PC and a 400 m or 800 m race for anaerobic glycolysis.
• 💡Compare anaerobic and aerobic systems in terms of rate, yield and duration to answer 'explain' questions fully.
• 💡Anchor each point on the continuum to a named activity and its intensity and duration.
• 💡Use the phrase relative contribution to show you understand overlap rather than switching.
• 💡Check whether the question asks which system dominates or how contributions change, and answer that specific demand.
• 💡Name the activity, state its intensity and duration, then justify the dominant system.
• 💡For intermittent activities, explain how recovery periods change the contribution of each system.
• 💡Use comparative language such as predominantly or mainly rather than claiming exclusive use.
• 💡Link each fibre type to a specific ATP-generating system and a sporting example to show applied understanding.
• 💡Use comparative language such as 'higher mitochondrial density' or 'faster rate but lower yield' rather than listing isolated facts.
• 💡Check that any statement about ATP yield or rate is correctly paired with the relevant fibre type and energy system.
• 💡Name the specific energy system and give an approximate duration or sporting example to show precise knowledge.
• 💡Use correct terminology such as 'phosphocreatine', 'ADP', 'anaerobic glycolysis' and 'lactate' rather than vague terms like 'energy'.
• 💡Explain why the system is suited to short duration, high intensity exercise by linking speed of ATP regeneration to intensity and fatigue.
• 💡Learn the two components by name and by fuel so you can match each to a sporting example.
• 💡Use precise terms such as resynthesise, phosphocreatine, glucose, pyruvate and lactate rather than vague words like energy stuff.
• 💡When a question gives a duration or intensity, decide which anaerobic component fits best before choosing an answer.
• 💡Link the ATP-PC system to very short, very intense actions such as a sprint start or a jump.
• 💡Use the correct enzyme name, creatine kinase, and the correct breakdown products, creatine and phosphate.
• 💡If a question asks why performance drops in repeated sprints, explain that phosphocreatine stores are depleted and need oxygen to recover.
• 💡Link each term to a measurable feature: lactate threshold to intensity, OBLA to a blood lactate concentration such as 4 mmol·dm⁻³, and capacity to sprint performance.
• 💡Use a concrete event such as 400 m to show how lactate accumulation and producing capacity affect performance.
• 💡Distinguish clearly between lactate production, accumulation and clearance when explaining fatigue.
• 💡Name the stages: glycolysis, Krebs cycle and electron transport chain, and state that oxygen is available.
• 💡Use a concrete event such as marathon running to show how fat oxidation and glycogen sparing support performance.
• 💡Link intensity to the lactate threshold to explain why lactate does not accumulate during steady aerobic work.
• 💡Link each stage to its location: sarcoplasm for glycolysis, mitochondrial matrix for the link reaction and Krebs cycle, inner mitochondrial membrane for the electron transport chain.
• 💡Use the phrase final electron acceptor when explaining oxygen's role, and state that water is formed.
• 💡Compare fuel use by intensity: fat dominates at low intensity, carbohydrate contributes more as intensity rises within the aerobic range.
• 💡When a question asks why the aerobic system suits prolonged exercise, refer to its large ATP yield and lack of fatiguing by-products.
• 💡Sketch or describe the oxygen consumption curve: rapid rise at the start, plateau at steady state in submaximal work, continued rise to VO2 max in maximal work, then slow decline during EPOC.
• 💡Use the term steady state when oxygen consumption plateaus and ATP demand is met aerobically.
• 💡Link the size of the oxygen deficit to exercise intensity: higher intensity means a larger anaerobic contribution and a larger deficit.
• 💡Explain EPOC functions with specific examples, such as restoring phosphagen stores and clearing lactate.
• 💡Read the question carefully to check whether it asks about the fast component, the slow component or EPOC as a whole.
• 💡Use the correct units when a value is given: ml·kg⁻¹ for relative oxygen volume or L for absolute volume.
• 💡Link each component to its main recovery process rather than just naming it.
• 💡Eliminate options that describe the oxygen deficit or lactate production during exercise, as these are not EPOC.
• 💡Check whether the question asks for a factor that affects VO₂ max or for the definition of VO₂ max.
• 💡When a factor is given, link it to oxygen delivery or use rather than just naming it.
• 💡Remember that both genetic and training factors can affect VO₂ max, so avoid options that say only one matters.
• 💡Pay attention to the units provided: L/min for absolute VO₂ max and ml·kg⁻¹·min⁻¹ for relative VO₂ max.
• 💡Learn the unit conversion 1 kcal ≈ 4.2 kJ and be ready to convert between kJ and kcal.
• 💡Link each measurement method to its specific energy system: indirect calorimetry and VO₂ max tests for aerobic, lactate sampling for anaerobic.
• 💡Be prepared to identify indirect calorimetry, lactate sampling and VO₂ max tests as the specified methods for measuring energy expenditure.
• 💡Write the RER equation as VCO₂ ÷ VO₂ and practise interpreting values near 0.7, 0.85 and 1.0.
• 💡Link the energy equivalent of oxygen to the substrate: about 20 kJ per litre for fat and about 21 kJ per litre for carbohydrate.
• 💡When evaluating the method, mention both its laboratory accuracy and its practical limitations such as equipment and altered breathing.
• 💡Link each lactate value to the exercise intensity and timing at which it was taken, rather than quoting a number in isolation.
• 💡Use the term lactate threshold precisely: the intensity at which lactate accumulation begins to exceed clearance.
• 💡When comparing two performers, compare their lactate responses at the same relative intensity to make the comparison valid.
• 💡State the equipment used (treadmill or cycle ergometer, mouthpiece or mask, gas analyser) when describing the test.
• 💡Explain the plateau criterion clearly, as it is the key indicator that VO₂ max has been reached.
• 💡Use correct units and specify whether the value is absolute or relative to body mass.
• 💡Learn the formula and the two anchor values, 0.7 for fat and 1.0 for carbohydrate, so you can reason from any given figure.
• 💡When a question supplies an RER value, state the dominant fuel and justify it with the ratio rather than guessing.
• 💡Link RER to exercise intensity: as intensity rises, carbohydrate contribution increases and the ratio moves towards 1.0.
• 💡Name the training method, identify the energy system it targets, then state the specific adaptation it produces.
• 💡Use the performer's event to justify the method rather than describing the method in isolation.
• 💡Refer to work-to-rest ratios and intensity when explaining why a method stresses one system more than another.
• 💡Link each adaptation to the aerobic energy system by explaining how more oxygen reaches the working muscles.
• 💡Use the term partial pressure of oxygen rather than saying there is less oxygen in the air.
• 💡Describe the live high, train low method to show understanding of how altitude training is applied in practice.
• 💡Name the energy systems used during the work intervals and explain why they dominate at high intensity.
• 💡Give a concrete example of a work-to-recovery ratio, such as 30 seconds of sprinting followed by 60 seconds of walking.
• 💡Explain how the recovery interval supports performance in the next work interval.
• 💡Link each phase of the stretch-shortening cycle to what happens in the muscle and tendon, not just to the name of the exercise.
• 💡When a question asks why plyometrics improves power, refer to both stored elastic energy and the stretch reflex.
• 💡Use a named example such as a depth jump and state which energy system fuels it.
• 💡Define each of the three components separately before explaining how a drill develops them.
• 💡Name a specific drill and state which component it mainly targets.
• 💡Link the intensity and duration of SAQ work to the energy system that fuels it.
• 💡Always use specific sporting examples to illustrate energy system use. For example, a 100m sprint (ATP-PC), a 400m race (glycolytic), and a marathon (aerobic).
• 💡When explaining fatigue, link the by-products to their effects: lactic acid lowers pH, inhibiting enzymes; PC depletion reduces ATP resynthesis; glycogen depletion causes hypoglycaemia.
• 💡Use correct terminology: 'resynthesis' not 'production' of ATP; 'anaerobic' not 'without oxygen'; 'predominant' not 'only' energy system.

• Thinking only one energy system works at a time. Correction: all three contribute, but the dominant system shifts with intensity and duration.
• Believing lactate is a waste product that causes all fatigue. Correction: lactate is a fuel that can be oxidised or converted, and fatigue has multiple causes.
• Confusing the ATP-PC system with anaerobic glycolysis. Correction: ATP-PC uses phosphocreatine for very short bursts, while glycolysis breaks down glucose or glycogen.
• Assuming recovery only means resting. Correction: recovery involves active restoration of ATP and PC stores, lactate clearance and elevated oxygen consumption.
• Thinking ATP is stored in large amounts. Correction: ATP stores are very limited and must be continuously resynthesised.
• Believing energy transfer only happens in the muscles. Correction: it occurs in cells throughout the body, with muscle cells having high demand.
• Confusing the fuel for anaerobic glycolysis with fat. Correction: glycolysis uses glucose or glycogen, while fat is used aerobically.
• Assuming the aerobic system produces lactate. Correction: lactate forms when oxygen supply cannot meet demand, so it is associated with anaerobic glycolysis.
• Thinking glycolysis happens in the mitochondrion: it occurs in the sarcoplasm, and only pyruvate and the later stages are mitochondrial.
• Believing oxygen directly makes ATP: oxygen is the final electron acceptor at the electron transport chain, and ATP is made by oxidative phosphorylation.
• Confusing beta oxidation with glycolysis: beta oxidation breaks down fatty acids, not glucose, into acetyl coenzyme A units.
• Assuming the Krebs cycle produces most of the ATP: it mainly reduces NAD and FAD, and the electron transport chain generates most ATP.
• Saying the ATP-PC system uses oxygen: it is anaerobic and relies on phosphocreatine breakdown.
• Believing lactate itself is the sole cause of fatigue: hydrogen ion accumulation is the key contributor, and lactate is a by-product.
• Thinking anaerobic glycolysis occurs in the mitochondrion: it takes place in the sarcoplasm.
• Confusing the duration of the two systems: ATP-PC lasts only a few seconds, while anaerobic glycolysis extends to about two minutes.
• Believing the systems work one after another in strict sequence: correct this by stating they overlap and contribute simultaneously, with only the dominant contribution changing.
• Thinking a single system supplies all ATP at any moment: correct this by describing a relative contribution that shifts along the continuum.
• Assuming duration alone decides the dominant system: correct this by linking the shift to both intensity and duration together.
• Judging the dominant system from duration alone: correct this by weighing intensity and duration together.
• Treating intermittent team games as purely anaerobic: correct this by explaining that the aerobic system supports recovery and sustained periods.
• Ignoring recovery between efforts: correct this by stating that recovery allows ATP-PC resynthesis and reduces fatigue.
• Confusing continuous high-intensity events like a 1500 m race with intermittent activities: correct this by noting the 1500 m is continuous and relies on the aerobic system for sustained work.
• Thinking fast twitch fibres produce more total ATP than slow twitch fibres; correction: they produce ATP faster, but aerobic slow twitch fibres yield more ATP per glucose and sustain production longer.
• Believing all fast twitch fibres are identical; correction: type IIa fibres have a greater oxidative capacity than type IIb/IIx fibres.
• Confusing the phosphocreatine system with aerobic respiration; correction: the PC system is anaerobic and regenerates ATP rapidly without oxygen, whereas aerobic respiration requires oxygen.
• Assuming slow twitch fibres cannot produce ATP quickly; correction: they can, but at a lower rate than fast twitch fibres, which is why they suit endurance rather than sprinting.
• Thinking aerobic respiration is the main ATP source in a 100 m sprint; correction: the PC system and anaerobic glycolysis dominate because they do not require oxygen and work faster.
• Believing lactate is the cause of all fatigue; correction: fatigue in short high intensity exercise is multifactorial, including PC depletion and accumulation of metabolites such as hydrogen ions.
• Confusing the duration of the PC system with that of anaerobic glycolysis; correction: PC lasts about 10 seconds, while glycolysis can supply ATP for up to roughly two minutes.
• Assuming oxygen is completely absent; correction: oxygen is present but its delivery and use are too slow to meet the ATP demand at maximal intensity.
• Thinking anaerobic means no energy is released: correction — anaerobic means no oxygen is used, but ATP is still resynthesised and energy is released.
• Confusing the two anaerobic components: correction — the ATP-PC system uses phosphocreatine, while the lactic acid system uses glucose breakdown to lactate.
• Believing lactate is a waste product with no further role: correction — lactate can be transported to other muscles, the heart and the liver and used or converted, and it is linked to fatigue when it accumulates.
• Assuming the anaerobic system only works at rest: correction — it dominates during short, high-intensity exercise and at the start of all exercise.
• Saying the ATP-PC system uses glucose: correction — it uses phosphocreatine, not glucose.
• Believing it can fuel a whole match: correction — phosphocreatine stores last only a few seconds of maximal effort.
• Thinking lactate is produced by this system: correction — the ATP-PC system produces no lactate; lactate comes from the lactic acid system.
• Ignoring recovery: correction — phosphocreatine is rebuilt using oxygen after exercise, which is why repeated sprints slow down.
• Thinking the anaerobic glycolytic system produces large ATP amounts like the aerobic system; correction: it yields only a small net ATP per glucose and is suited to short, intense efforts.
• Confusing lactate threshold with OBLA; correction: lactate threshold is the intensity where lactate begins to accumulate faster than clearance, while OBLA is a defined blood lactate concentration.
• Believing lactate itself directly causes all fatigue; correction: lactate accumulation lowers pH, and the resulting acidity interferes with enzyme activity and muscle contraction.
• Assuming the system only works without any oxygen at all; correction: it operates when oxygen supply cannot meet demand, and lactate is also cleared and used as fuel.
• Thinking the aerobic system produces ATP quickly; correction: it resynthesises ATP more slowly than anaerobic systems but sustains work for long periods.
• Assuming only carbohydrate is used; correction: fats contribute increasingly as exercise duration extends, sparing glycogen.
• Believing lactate always accumulates in aerobic exercise; correction: below the lactate threshold, lactate is cleared as quickly as it is produced.
• Confusing long duration lower intensity exercise with sprint events; correction: sprint and power events rely mainly on anaerobic systems, while long duration lower intensity work relies on aerobic transfer.
• Thinking the aerobic system only uses fat: it also uses carbohydrate, and carbohydrate is the main fuel at higher submaximal intensities.
• Believing glycolysis happens in the mitochondria: glycolysis occurs in the sarcoplasm, while the Krebs cycle and electron transport chain occur in the mitochondria.
• Stating that hydrogen ions pass along the electron transport chain: electrons pass along the chain, while hydrogen ions are pumped across the membrane.
• Confusing the aerobic system with the ATP-PC system: the ATP-PC system is anaerobic and lasts only a few seconds, whereas the aerobic system sustains prolonged exercise.
• Confusing oxygen deficit with EPOC: the deficit occurs at the start of exercise, while EPOC occurs after exercise.
• Thinking oxygen deficit only happens in maximal exercise: a deficit occurs whenever exercise begins, but it is larger in maximal work.
• Believing steady state means no anaerobic contribution at all: at steady state, aerobic metabolism meets most ATP demand, but the initial deficit still had to be covered anaerobically.
• Treating VO2 max as the oxygen consumed at any intensity: VO2 max is the highest rate of oxygen uptake achievable during maximal exercise.
• Confusing EPOC with the oxygen deficit: EPOC occurs after exercise, whereas the oxygen deficit occurs at the start of exercise before steady state is reached.
• Thinking EPOC is only about lactate removal: the fast component restores PC and myoglobin oxygen, while the slow component deals with lactate and other recovery processes.
• Believing EPOC ends as soon as breathing returns to normal: ventilation may normalise before the slow component is complete, so EPOC can continue for hours.
• Stating that EPOC is a rate of oxygen consumption: it is a volume of oxygen consumed above resting baseline, measured in L or ml·kg⁻¹, not ml·kg⁻¹·min⁻¹.
• Confusing VO₂ max with lung volume: VO₂ max depends on oxygen uptake, transport and use, not just how much air the lungs can hold.
• Thinking VO₂ max is fixed and cannot change: training can improve it, although genetics set an upper limit.
• Believing VO₂ max is the same for everyone of the same age: sex, body composition and training status also cause differences.
• Assuming VO₂ max can only be measured in one way: it can be absolute (L/min) or relative to body mass (ml·kg⁻¹·min⁻¹).
• Thinking energy expenditure is measured only in kcal: the SI unit is the kilojoule (kJ), and kcal is a common alternative (1 kcal ≈ 4.2 kJ).
• Confusing the purpose of lactate sampling: it measures anaerobic energy expenditure and OBLA, not aerobic capacity.
• Assuming VO₂ max tests only measure oxygen uptake: they are also a key method for evaluating maximum aerobic energy expenditure.
• Reversing the RER formula: RER is carbon dioxide produced divided by oxygen consumed (VCO₂ ÷ VO₂), not the other way round.
• Believing RER above 1.0 means fat is the main fuel: a value above 1.0 reflects a heavy anaerobic contribution and increased carbon dioxide from buffering.
• Treating indirect calorimetry as measuring heat directly: it estimates expenditure from respiratory gases, whereas direct calorimetry measures heat loss.
• Thinking lactate is the direct cause of the burning sensation and of delayed onset muscle soreness; correction: lactate is a metabolic by-product that is cleared and reused, and the burning sensation relates to other factors such as hydrogen ion accumulation.
• Believing lactate only appears when the body has run out of oxygen; correction: lactate is produced continuously, and it accumulates when production exceeds clearance, which can occur even when oxygen is available.
• Assuming a single lactate reading gives a complete picture of fitness; correction: lactate must be interpreted against the exercise intensity and the sampling time, and often several samples across intensities are needed.
• Confusing lactate with lactic acid as if they were identical in the blood; correction: at blood pH, lactate exists largely as the lactate ion, and lactate sampling measures lactate concentration.
• Thinking VO₂ max is the maximum heart rate; correction: VO₂ max is a measure of oxygen uptake, while heart rate is a separate cardiovascular variable.
• Assuming the test ends as soon as the performer feels tired; correction: the test continues with progressive increments until oxygen uptake plateaus or the performer cannot continue, so that a true maximum is approached.
• Believing VO₂ max can be measured accurately without a gas analyser; correction: direct measurement requires analysis of expired air, although submaximal or field tests can estimate it indirectly.
• Confusing absolute and relative VO₂ max units; correction: absolute values are given in dm³·min⁻¹, while relative values are given per kilogram of body mass, such as cm³·kg⁻¹·min⁻¹.
• Reversing the ratio and writing VO₂ ÷ VCO₂: the correct form is carbon dioxide produced divided by oxygen consumed.
• Assuming RER always equals exactly 1.0 during hard exercise: values can exceed 1.0 because of hyperventilation and buffering.
• Treating RER as a direct measure of fat or carbohydrate burned in a single muscle: it is a whole-body estimate of substrate use.
• Confusing RER with RQ (respiratory quotient) measured at tissue level: RER is measured at the mouth from exchanged gases.
• Assuming any training improves all energy systems equally: the method, intensity and work-to-rest ratio determine which system adapts most.
• Believing high-intensity interval training only improves the ATP-PC system: it also stresses the glycolytic system and can develop aerobic capacity.
• Thinking continuous training is useless for team-sport players: it builds the aerobic base that supports recovery between high-intensity efforts.
• Ignoring recovery duration: short recoveries keep stressing the glycolytic system, while long recoveries allow ATP-PC resynthesis.
• Thinking altitude air contains less oxygen overall: the percentage of oxygen is similar, but the partial pressure of oxygen is lower, so less oxygen diffuses into the blood.
• Believing altitude training directly increases muscle strength: the main adaptation is increased red blood cell production and oxygen transport, which supports aerobic performance.
• Assuming athletes should perform all training at altitude: living high but training low preserves training intensity and technique while still gaining blood adaptations.
• Confusing EPO with a training method: EPO is the hormone released by the kidneys that stimulates red blood cell production.
• Thinking HIIT only trains the aerobic system: the intense work intervals rely heavily on the ATP-PC system and anaerobic glycolysis, although aerobic capacity also improves.
• Believing recovery intervals should be complete rest: light active recovery helps remove by-products and maintain blood flow, though the key point is that recovery is brief.
• Assuming HIIT is suitable for all athletes at all times: it is demanding and should be periodised, with adequate recovery to avoid overtraining.
• Confusing HIIT with continuous steady-state training: HIIT uses repeated short intense efforts separated by recovery, not one sustained submaximal effort.
• Thinking plyometrics trains aerobic endurance: it develops explosive power through short, intense efforts, so it relies on the ATP-PC system rather than aerobic metabolism.
• Believing the amortisation phase should be long: a long pause lets stored elastic energy dissipate as heat, so the amortisation phase should be as short as possible.
• Confusing the eccentric phase with the concentric phase: the eccentric phase is the rapid stretching of the muscle as it lengthens under tension, while the concentric phase is the forceful shortening that produces the jump.
• Treating speed, agility and quickness as the same quality: speed is straight-line movement over a distance, agility involves controlled changes of direction, and quickness is explosive reaction over short distances.
• Assuming SAQ training is mainly aerobic: the efforts are short and intense, so the ATP-PC system dominates, with anaerobic glycolysis contributing during repeated bouts.
• Thinking agility is only about physical speed: it also requires balance, control and the ability to react to a stimulus such as an opponent or a ball.
• Students often think the ATP-PC system produces lactic acid. Correction: The ATP-PC system produces creatine and inorganic phosphate, not lactic acid. Lactic acid is a by-product of the anaerobic glycolytic system.
• Students believe that only one energy system is used at a time. Correction: All three systems contribute simultaneously, but one is predominant depending on intensity and duration. For example, the aerobic system contributes even during short sprints.
• Students confuse the order of energy system recruitment. Correction: The ATP-PC system is used first for explosive efforts, followed by the anaerobic glycolytic system, then the aerobic system as duration increases.

1. 1Day 1-2: Create a detailed comparison table of the three energy systems including fuel, duration, intensity, ATP yield, by-products, and sporting examples.
2. 2Day 3-4: Learn the stages of aerobic respiration (glycolysis, Krebs cycle, electron transport chain) and the ATP yield at each stage.
3. 3Day 5-6: Practice applying energy systems to different sporting activities, explaining the interplay and predominant system.
4. 4Day 7-8: Complete past paper questions on energy systems, focusing on 6-mark structured questions and data analysis.
5. 5Day 9-10: Review examiner reports and mark schemes to understand common pitfalls and refine exam technique.

• 📋Short answer questions (2-4 marks) asking for definitions, by-products, or duration of specific energy systems. Advice: Be precise with terminology and use correct units.
• 📋Structured 6-mark questions requiring explanation of energy system interaction during a specific sport. Advice: Use a clear structure: introduce the systems, explain the predominant system at each stage, and conclude with interplay.
• 📋Data analysis questions presenting graphs of ATP resynthesis or lactic acid concentration. Advice: Identify the dominant system from the data and explain the trends using physiological knowledge.
• 📋Evaluation questions (9-15 marks) on training programmes to improve specific energy systems. Advice: Link training methods (e.g., interval training, continuous training) to energy system adaptations and justify choices.